DIGITAL DIFFERENTIAL RELAYS FOR TRANSFORMER PROTECTION USING WALSH SERIES AND LEAST SQUARES ESTIMATORS

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DIGITAL DIFFERENTIAL RELAYS FOR TRANSFORMER
PROTECTION USING WALSH SERIES AND LEAST SQUARES
ESTIMATORS

• Ali Reza FEREIDUNIAN,
• Mansooreh ZANGIABADI,
• Majid SANAYE-PASAND,
• Gholam POURNAGHI
• ECE Dep., Faculty of Engg., University of
  Tehran,Tehran, IRAN
• Kerman Regional Electric Company (KREC),
  Kerman, IRAN

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Differential Protection

• The fundamental principle of differential
  protection sum of the currents entering a device
  through normal paths should be zero Kirchhoff's
  Current Law (KCL).
• If the currents enter (or leave) through abnormal
  paths, namely fault paths, then the sum of the
  currents through normal paths will not be zero.

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Differential Protection Illustration
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Problems in transformer differential protection

• inrush current,
• CT inaccuracy,
• CT saturation,
• over-excitation.
• These problems produce fault trips (fault alarm
  when there isnt any trip) or no alarm when there
  is a trip in transformer protection function

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DIFFERENTIAL RELAY IMPLEMENTATION

• Current Sensor (CT) converts large amounts of
  current to small amounts
• Data Acquisition System gathering data
• Filter anti aliasing
• Pre-processor scaling and so on
• Estimator estimating peak phase
• Decision Maker (Classifier) fault/no fault

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Effect of CT Saturation on a Sinusoidal Current
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WE HAVE USED TWO METHODS

• FOR ESTIMATING PEAK AND PHASE OF INPUT WAVE.

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Walsh coefficients

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Walsh Series (Ctd)

• WA F
• FA-1W
  where
• F F0 F1 F2 F3 F4 F5 F6 F7 F8
• A-1AT

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Least Squares

• AX B
• E AX B
• LPI(A) B
• LPI(A)

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Sampling

• 12 point window (for half cycle estimation) or
• 24 points (for full cycle estimation)
• with
• 24 sample/cycle sampling system

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Least square frequncy response for fundamental
frequency
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The Decision Space
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Inrush Pattern Recognition

• A significant second harmonic
• Inrush Current Pattern Recognition

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A CASE STUDY

• Real recorded data
• Transformer internal fault,
• Transformer external fault,
• Transformer inrush current

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High and Low Voltage Side Currents for External
Fault
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High and Low Voltage Side Currents for Internal
Fault
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High and Low Voltage Side Currents for Inrush
Current
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Three Phases Differential Currents in External
Fault
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. Three Phases Differential Currents in Internal
Fault
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Three Phases Differential Currents in Inrush
Current
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Decision Space in External Fault for three Phases
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Decision Space in Internal Fault for Three Phases
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Decision Space in Inrush Current for Three Phases
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Second/Fundamental Harmonic Ratio for External
Fault
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Second/Fundamental Harmonic Ratio for Internal
Fault
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Second/Fundamental Harmonic Ratio for Inrush
Current
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General Trip Alarm for External Fault
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General Trip Alarm for Internal Fault
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General Trip Alarm for Inrush Current
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Summary

• A digital differential relay for transformer
  protection was presented.
• Two estimator systems Walsh series and least
  squares algorithms were formulated and designed.
• The differential protection decision maker
  subsystem was introduced.
• Current signals harmonic components and second
  harmonic restraint concept were utilized in
  decision maker subsystem.

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Conclusion

• In a practical case study, the designed relay
  performance was tested under three real
  circumstances external fault, internal fault and
  inrush current.
• It was shown -using graphs and illustrations-
  that the presented relay issues trip alarm for
  transformer internal fault, and does not issue
  trip alarm for external fault and inrush current
  situations.

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Conclusion (Ctd)

• It were seen that both estimation algorithms
  perform their job correctly.
• Walsh series acts better than least squares
  algorithm, especially on second harmonic
  estimation.
• An anti alias filter (for example a Butterworth
  one) will improve response of the estimator.